The invention relates to fuel cells and, more particularly, to a system and method for operating a fuel cell stack assembly which assists in start up at sub-freezing temperatures.
Fuel cells are useful as an alternative for converting hydrogen containing fuel and oxidant into energy. Such fuel cells have many desirable applications, some of which involve exposing the fuel cell to sub-freezing conditions.
A typical fuel cell, especially a typical PEM fuel cell, includes a series of fuel cell assemblies arranged in a stack which defines end fuel cells. Opposite to each end fuel cell, end plate/current collectors are typically positioned which are relatively large-mass structures.
Water is generated during operation of a fuel cell and, when operation of a fuel cell is stopped, this water can freeze when exposed to sub-freezing temperatures. Freezing of this water can be extremely problematic depending upon where the water is when it freezes.
One approach has been to allow the cell to freeze only after all water has been drained except that water which resides in the pores of the water transfer plates, in the membrane electrode assemblies, and a small undrainable amount of water in the cell substrates. Such a cell will start without difficulty because heat released during initial operation is sufficient to melt the water in the water transfer plates before water production exceeds the capacity of the substrates to store the product water without blocking oxidant access to the catalyst. Once the water transfer plate water has melted, pressure difference between the oxidant compartments and the cooling compartments provides means to drive water from the catalysts, through the substrates and pores in the water transfer plate, and into the virtually limitless reservoir provided by the empty coolant channels. This permits the cell stack assembly to self-heat to normal operating temperature.
One persisting problem, however, is with the end cells. The thermal mass of pressure plates or current collectors at the end of the stack prevents melting of frozen water in the water transfer plate pores before capacity of the substrates to absorb the product water is exceeded. Consequently, oxidant access to the catalyst layer in these end cells is eliminated. In the process, water is forced into the catalyst layer and bi-layer pores with sufficient force to prevent easy withdrawal once all of the cells in the stack reach normal operating temperature.
Clearly, a solution to this problem is needed.
It is therefore the primary object of the present invention to provide a system and method for facilitating startup from sub-freezing conditions.
Other objects and advantages of the present invention will appear herein below.